Although the present disclosure is particularly suited to unducted-fan turbine engines, the implementation thereof is however not limited to such an application.
As is known, an unducted-fan turbine engine may comprise two coaxial contra-rotating external propellers, respectively upstream (front) and downstream (rear), which are each rotated by a turbine and extend substantially radially outside the nacelle of the turbine engine. Each propeller usually comprises a hub concentric with the longitudinal axis of the turbine engine, on which blades are fixed.
The aerodynamic interaction between the upstream and downstream contra-rotating propellers of such an unducted-fan turbine engine causes very high operating acoustic levels. This is because the rotation of the upstream and downstream contra-rotating propellers causes, among other things, the formation of:                wakes along the span of the blades, downstream thereof;        main vortices at the free end of the blades.        
These aerodynamic disturbances downstream of the upstream propeller are partly the cause of the interaction aerodynamic noise when they strike the downstream propeller or pass close thereto.
In particular, during phases of low-speed operation of an unducted-fan turbine engine (such as, when it is mounted on an aircraft, takeoff, the climbing phase, landing and approach), the dominant contribution of the radiated noise comes from the interaction lines associated with the downstream propeller that functions in the stream of the upstream propeller, passing through the vortex layers consisting of wakes and main vortices formed by the blades of the upstream propeller (also referred to as upstream blades). When a marginal vortex of upstream blades interacts with the blades of the downstream propeller (otherwise referred to as downstream blades), the interaction between downstream blade and marginal vortex dominates the acoustic spectrum radiated for the majority of the directivities.
Thus, in order to reduce the undesirable noise emissions of such turbine engines and thus meet the acoustic certification criteria imposed by the aviation authorities, it is necessary to reduce the low-speed radiated noise by reducing the interaction between downstream blade and marginal vortex.
Currently, the most widespread known solution—referred to as clipping—consists of reducing the diameter of the downstream propeller so as to make the main vortices generated by the upstream blades pass outside the downstream blades in order to limit the interaction of the latter with the main vortices. This generally involves an increase in the chord of the downstream blades in order to maintain the desired traction and the torque ratio between the upstream and downstream propellers. Such a solution may be pushed to the extreme by very highly loading the end of the upstream blades, so as to relieve the remainder of each of the upstream blades in order to reduce the impact of the wake of the upstream propeller on the downstream propeller, also giving rise to undesirable interaction noise.
However, such a solution proves to be acceptably only for an isolated configuration of the turbine engine (that is to say without any external element connected thereto) and without incidence. In the presence of elements (strut, fuselage) or incidence, the contraction and the axisymmetry of the flow of air behind the upstream propeller are modified, so that the clipping carried out no longer prevents the interaction of the downstream blades and the main vortices generated by the upstream blades. A greater reduction in the height of the downstream blades (corresponding to significant clipping) involves an increase in the chord associated with the downstream blades so as to preserve the load, which degrades the efficiency of the associated turbine engine and is therefore not satisfactory.
The applicant proposed another solution to this problem, in the prior application FR 2 980 818. This other solution consists of equipping each upstream blade with a single protrusion on its leading edge, this protrusion being situated in a predetermined location in order to locally disturb, when the propeller rotates, the distribution of the circulation around each blade, so as to form two independent main vortices downstream:                a first natural vortex (or marginal vortex) forming at the free end of the blade;        a second distinct forced vortex (or supplementary main vortex) taking place in the vicinity of the protrusion.        
The marginal and supplementary vortices are co-rotating (that is to say they have the same direction of rotation) and remain independent of each other as far as the downstream propeller. In this way a modification to the distribution of the circulation around the single local position is modified and the result is the formation of two vortices—of lower intensity than the single marginal vortex observed in the prior art—that do not merge together.
However, the applicant found that this other solution is not entirely satisfactory since it is not effective irrespective of the operating conditions, that is to say the various flight phases (takeoff, cruising, landing, etc.). This is because the rotation speed of the upstream propeller, the speed of travel of the aircraft equipped with this propeller, and the pitch angle of the blades of this propeller for example, have an influence on the path of the vortices from the leading edges of the blades. In the solution proposed in the prior application FR 2 980 818, the position of the protrusion on each blade is fixed and determined for a single flight phase, preferably takeoff, in order to reduce the noise nuisance for people living near the airport.